Selective amplifier



Jan. 10, 1933. A. 5. ms 1,893,563

SELECTIVE AMPLIFIER Filed June 8, 1929 lNV/EgJO $2 'm (JATTORNEYPatented Jan. 10, 1933.

UNITED STATES PATENT eerie ALGER S. RIGG-S, OF NEW YORK, N. Y., ASSIGNORTO WILLIS L. PRATT, OF NEW YORK, N. Y., FRANK W. MCDONELL, 0F EYE, NEWYORK, AN D FREDERICK R. RICH, OF NEW YORK, N. Y.

SELECTIVE AMPLIFIER Application filed June 8,

This invention relates to amplifiers of electrical variations whereinone of the component parts is an electron-discharge device, but moreespecially the invention relates to selective amplifiers such as areused for instance in transmission and reception of radio telegraphy andtelephony, or in any other application in which it is desired to amplifya specific frequency or band of frequencies.

An especial object of this invention is the provision of an amplifiersystem capable of amplifying a predetermined frequency to the exclusionof other frequencies.

A further object of the present invention is the reduction of theeifective electro-sta-tic capacity between grid and plate by loweringthe resistance into which the plate circuit is connected, therebyeliminating to a large exicent the tendency to feed-back and oscilate.

A still further object of the present invention is the provision of aselective amplifier in which the signal static ratio is considerablyincreased.

Figure l is a diagrammatic drawing of an electron-discharge device Vwhich may be of the conventional three electrode type or may be of thetype described in my co-pending ap plication Serial No. 358,774, filedApril 29, 1929, and/or my co-pending application Serial No. 363,201,filed May 15, 1929. The battery A serves to maintain the filament f at atemperature necessary to produce sufficient electron emission from thecathode 0. The ionization shield s surrounds the cathode as in myco-pending application Serial N 0. 358,774, filed April 29, 1929. Thebattery B serves to supply voltage for the plate-cathode circuit. Asource of input potential, such as for instance a tuned radio-frequencytransformer T, is connected at its opposite ends to the cathode c andthe grid g. The output or load circuit consists of the resister Ra?shunted by the tuned circuit LC, points 1 and 2 thereof constituting theoutput terminals which may be connected to a detector or to the inputterminals of an amplifier such as herein disclosed if more than onestage of amplification is desired.

In accordance with the well-known action 1929. Serial No. 369,383.

of the three electrode electron-discharge device, variations of thepotential of the grid 9 will cause a current to flow in accordancetherewith through the circuit including the plate-cathode path.

This invention anticipates the use of a resonant circuit consisting ofinductance and capacity, either one of which or both may be variable(herein the capacity is shown as the variable element) as a portion ofthe platecircuit load. In contra-distinction to conventional practice, Iintroduce a current into a series resonant circuit instead of apotential into a parallel resonant circuit.

In order to more readily understand the advantages of this invention, itmust be realized that the effective impedance between the oppositeterminals of a series resonant circuit (to resonant frequency) is equalto the value of the resistance included between the opposite terminals.The total voltage in troduced into such a series resonant circuit dropsacross this resistance, shown as B and with a given current thepotential across the inductance L and the condenser C may be manythousand times greater than the voltage introduced between the oppositeterminals of the series resonant circuit.

In accordance with the above, I pass the alternating component of theplate circuit through a load consisting of the resister R00 shunted bythe series resonant circuit L-R,;C. At the resonant frequency theeffective impedance between points 1 and 3 is equal to Ron shunted onlyby R since i the reactances of L and C are equal and opposite. Duringthis condition, if R is equal to B which is the optimum, one half of theA. C. plate current will flow through R00, while the other half willflow through the series resonant circuit above referred to, and althoughthe potential across R00 may be extremely small, the potential acrosseither the inductance or the capacity will be equal to the product ofthe A. C. current flowing therethrough, and the reactance of thecondenser or inductance. It is therefore evident that a relatively highdegree of amplification may be obtained with this system, coincidentlywith a high degree 'of selectivity and transient disturbance, such asfor instance atmospheric disturbances, is twice that of fered to thesignal frequency (if R00 is equal to R It is generally believed that theobjectionable noises due to atmospheric disturbances in radio receivingsystems are caused principally by impulse excitation of the resonantcircuit. In other words, a transient disturbance such as static causesthe tuned circuit to oscillate for a few cycles at the frequency towhich it is tuned, the amplitude of the oscillation and the timeduration thereof being dependent upon the decrement or effectiveresistance of, the tuned circuit. By use of the present system,.I amenabled to secure the same degree of signal selectivity as withconventional parallel resonant circuits, co-incidently with a very highand adjustable decrement or resistance to transient response.

If, for instance, R00 is made a variable resistance, the response tosignal may be increased co-incidently with an increase in the decrementoffered to transient disturbances, but at slight sacrifice of signalselectivity. Inasmuch as this invention utilizes the electron-dischargedevices as current and not as voltage generators, electron-dischargedevices having a low value of plate resistance and a high value ofmutual conductance are desirable. Since only. the mutual conductancecontributes toward the degree of amplification obtained in the presentsystem, it is unnecessary and inadvisable to utilize electron-dischargedevices having a high value amplification factor.

In order to utilize to best advantage the high decrement which thissystem offers to transient impulses when used in radio receivingsystems, the antenna should be connected to the grid of theelectron-discharge device aperiodically and not by a tuned transformeras shown on the drawin The amplifier herein shown and described consistsof only one stage but more than one stage may be employed wherenecessary and desirable, and although I have herein shown batteries aspower supply sources, it is understood that such power may be suppliedin other ways, as for instance, from commercial power circuits with thenecessary rectifiers and filters.

Although I have herein shown and described but one form of my presentinvention, I do not limit myself thereto, inasmuch as modifications andchanges may be made therein without departing from the original spiritof the invention or the scope of the appended claims.

What I claim is:

1. A selective amplifier of electrical variations consisting of anelectron-discharge device, a suitable source of input signal poten tial,power supply sources for cathode and anode circuits, and an anode outputnet-work consisting of a non-inductive resister shunted by a condenserand an inductance forming a series circuit resonant to the frequency ofthe input signal potential, said non-inductive resister having a valueapproximating that of the tuned circuit resistance at signal frequency.

2. A coupling medium for selective electron-discharge amplifiersconsisting of a noninductive resistance shunted by a series resonantcircuit, said non-inductive resistance having a value approximating thatof the tuned circuit resistance at signal frequency output circuit ofpreceding electron-discharge device eing connected across thenon-inductive resistance and input circuit of succeedingelectron-discharge device-being connected across one of the elementsforming the above mentioned series resonant circuit.

3. A selective output net-work for electrondischarge amplifiersconsisting of a non-inductive resistance adapted to be connected inseries with the plate-cathode path of an electron-discharge device and aseries resonant circuit in parallel with the non-inductive resistance,said non-inductive resistance hav- .f

ing a value approximating that of the tuned circuit resistance at signalfrequency.

7 4. In an amplifier of electrical variations,

selective to a particular frequency, one or more electron-dischargedevices, suitable I power supply sources therefor, and a coupling mediabetween said electron-discharge devices consisting of a non-inductiveresister shunted by a series resonant circuit, said noninductiveresister having a value approximating that of the tuned circuitresistance at signal frequency and being connected in series withpreceding plate-cathode circuit, and grid-cathode of succeedingelectron-discharge device being connected across a portion of saidresonant circuit.

In testimony whereof, I have signed my name to this specification.

ALGER S. RIGGS.

